The Portuguese, who is widely expected to succeed Louis van Gaal at the end of the season, sold the 22-year-old during his second spell at Stamford Bridge.
Lukaku's sale has proved a grave error of judgement based on his scintillating form this campaign, yet Mourinho can atone for his mistake by landing the Belgium international if he takes over at Old Trafford this summer.
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Despite having the uninspiring trio of Fernando Torres, Samuel Eto’o and Demba Ba at his disposal, one of Mourinho's first decisions upon returning to Chelsea in 2013 was to send Lukaku on loan to Everton, where he completed a £28 million move the following season.
And whilst the youngster blew hot and cold during his first official term at Goodison Park, the ex-Anderlecht striker has been sensational ever since and scored more goals this campaign (18) than Diego Costa, Loic Remy, Alexandre Pato and Radamel Falcao combined (14).
Mourinho was clearly wrong to let him leave but can now rectify his mistake if he replaces Van Gaal at the Theatre of Dreams.
Though Anthony Martial and Marcus Rashford have been revelations this campaign, United’s firepower is still lacking after the departures of Falcao, Robin van Persie and Javier Hernandez last summer.
Wayne Rooney’s form has waned and whilst the Red Devils have also been linked with a move for Paris Saint-Germain's Zlatan Ibrahimovic, Lukaku would represent terrific value for money in the long run.
Mourinho coaxed the best from heavyweights such as Didier Drogba and Diego Costa during his two spells in the capital, and Lukaku could thrive under his former manager if afforded consistent game time.
Any potential transfer would likely command a hefty transfer fee, yet the 22-year-old's form since moving to England suggests he could be the perfect signing to help propel United back to the top of English football.
Mourinho will be seeking redemption if he takes over and his former striker could be the man to salvage his managerial reputation and ensure he beats Pep Guardiola’s Manchester City to the league title next season.